1. Field of the Invention
The present invention relates to a circuit for driving an oscillatory-wave motor. More particularly, the present invention relates to a circuit for driving an oscillatory-wave motor, which causes a piezoelectric element to generate traveling waves by the expansion and contraction movement of the piezoelectric element and drives an oscillatory-wave motor by these traveling waves.
2. Description of the Related Art
In recent years, oscillatory-wave motors have come to be widely used in various fields for driving movable bodies which are in friction-contact with a surface of an elastic body by applying an alternating voltage of a given frequency to an piezoelectric element securely fixed to the elastic body and using the expansion and contraction movement of the piezoelectric element.
The oscillatory-wave motor is also called an ultrasonic-wave motor. As disclosed in, for example, Japanese Laid-Open Patent Application sho/59-96881, it operates on the principle that displacement of an elastic body is caused by mechanical resonance when the alternating voltage applied to a piezoelectric element matches a characteristic oscillation frequency of the elastic body; and this displacement is converted into elliptic vibrations on the surface of the elastic body.
The amplitude of the elliptic vibration is not directly proportional to the magnitude of the applied voltage (electrical field) but is strongly affected by the drive frequency of the applied voltage. Therefore, the characteristics of the oscillatory-wave motor depend upon the drive frequency of the applied voltage. Drive frequencies of individual oscillatory-wave motors differ according to the resonance frequency characteristic of the motor. In addition, the optimum drive frequency has characteristics such that the optimum point changes due to variations in loads; variations in the pressing force between the movable body and the elastic body of the oscillatory-wave motor; changes in the outside environment, such as temperature; changes in internal temperature caused by self-heat generation; or the like. For this reason, the drive frequency must be made to track the optimum point during the driving of the motor.
In connection with this, as disclosed in Japanese Laid-Open Patent Application sho/63-234881, the applicant of the present invention has proposed frequency automatic tracking means in which a piezoelectric element for feedback is securely fixed to an elastic body of an oscillatory-wave motor. Said motor outputs feedback signals by a piezoelectric-element effect when the oscillatory-wave motor is driven so that the orientation of deviations in the phase differences between the drive frequency signals of the oscillatory-wave motor and the feedback signals and the amount of deviations thereof are detected, said automatic frequency tracking means changing the oscillation frequency of a frequency oscillator on the basis of an error output with a predetermined value of phase deviation detecting means, which error occurs when the drive frequency is the optimum drive frequency in accordance with the phase deviation signal.
Since single driving in a short time and high-speed response are required in the oscillatory-wave motor, for example, for an actuator of a camera's auto-focus system, problems different from those occurring during continuous driving arise. In the phase difference detection method of the automatic frequency tracking means proposed in the above Japanese Laid-Open Patent Application sho/63-234881, it is repeated that the phase of the feedback signal is compared with that of the drive frequency signal at all times. A correction value is fed back to change the oscillation frequency. This circumstance is shown in FIG. 11. FIG. 11 shows a phase comparison output at the original oscillation output at a frequency division ratio of 1 and the correction. The finally corrected oscillation output moves to a stable point while repeating overshoot as shown in FIG. 11. To suppress this, the frequency division ratio may need to be made larger or the response slowed down by lowering the loop gain. Therefore, it takes a relatively long period of time before the frequency is stabilized, and high-speed response cannot be achieved.